Bifold door system

ABSTRACT

A bifold door system which includes a door frame having top and bottom walls. A trackway is mounted on one of the walls. A pair of door panels hinged together along adjacent vertical edges are associated with the frame with one of the panels being pivotally mounted at its top and bottom to the frame to enable the panel to rotate between a closed position substantially flush with the door frame and an open position out of the plane of the frame. The other panel has means thereon movably mounted in the trackway so as to permit the panel to be shifted between a closed position substantially flush with the door frame and an open position out of the plane of the frame. The hinged connection between the panel permits the panels to be opened to a substantially face-toface interengagement and to bias the panels into a closed position substantially flush with the door frame and maintain a continuing bias to retain the door panels in the closed position thereby requiring a predetermined force to open the door and permit passage through the door frame. Finally, resilient retention means are on the trackway to allow shifting of the hinged panels to the open position and then to retain the panels in substantially the open position until a predetermined force is applied at which time the retention means will release the panels and allow the bias means to automatically return the panels to the closed position.

United States Patent n91 Thun et al.

[ BIF OLD DOOR SYSTEM [75] Inventors: Leonard Thun, Bay Park; Leon David Grebow, Bronx, both of N.Y.

[73] Assignee: Acrite Industries, Inc., New York,

[22] Filed: Oct. 17, 1972 [21] Appl. No.: 298,333

[52] U.S. Cl. 160/118, 16/94 R [51] Int. Cl E06b 3/48 [58] Field of Search 160/118, 183,199, 206,

160/207; 16/93 R, 93 D, 94 R, 94 D, 95 R, 95 D, 96 R, 96 D, 180-184 Primary ExaminerPeter .M. Caun Attorney, Agent, or Firm-Kane, Dalsimer, Kane, Sullivan and Kurucz [57] ABSTRACT A.bifold door system which includes a door frame 14 May 21, 1974 having top and bottom walls. A trackway is mounted on one of the walls. A pair of door panels hinged together along adjacent vertical edges are associated with the frame with one of the panels being pivotally mounted at its top and bottom to the frame to enable the panel to rotate between a closed position substantially flush with the door frame and an open position out of the plane of the frame. The other panel has means thereon movably mounted in the trackway so as to permit the panel to be shifted between a closed position substantially flush with the door frame and an open position out of the plane of the frame. The hinged connection between the panel permits the panels to be opened to a substantially face-to-face interengagement and to bias the panels into a closed position substantially flush with the door frame and maintain a continuing bias to retain the door panels in the closed position thereby requiring a predetermined force to open the door and permit passage through the door frame. Finally, resilient retention means areon the trackway to allow shifting of the hinged panels to the open position and then to retain the panels in substantially the open position until a predetermined force is applied at which time the retention means will release the panels and allow the bias means to automatically return the panels to the closed position.

10 Claims, 8 Drawing Figures MIENTElJMAm ISM SHEET 1 [IF 3 "ATENTED HAY 2 1 I974 sum 2 at 3 BIFOLD DOOR SYSTEM BACKGROUND OF THE INVENTION In available bifold door systems, there are several disadvantages which often make them awkward and unwieldy to employ. Naturally it is desirable to provide a door, which when open, will remain in the open position and permit easy access through the door frame. The available devices which prevent the door from closing are usually of the locking type which are often quite complex and difficult to release in order to permit closing of the door when desired. It certainly would be advantageous to have a retention means to easily and efficiently retain the door in the open position when it is opened and which will release efficiently with a minimum amount of effort to permit the door to close upon demand.

Another difficulty which often arises with bifold doors is in retaining the doors in a positive closed position within the door frame. Doors which hang partially open could interfere with normal activity around the door area. Furthermore for many other obvious reasons it is not advantageous to have a partially open doorway or to have a door which will open upon minimal contact. A positive flush closing structure in a bifold door would solve readily apparent shortcomings in the field.

SUMMARY OF THE INVENTION With the above background in mind, it is among the primary objectives of the present invention to provide a bifold door system which permits easy, quick and efficient opening of the door with automatic retention means holding the door in an open position and permitting quick and efficient release of the holding action so that the door can return to the closed position. Automatic means are provided to permit the door to be manually opened to the fully open position and then to automatically return the door to the closed position upon release of the retention means. Furthermore the doors are automatically retained in a flush positive closed position within the door frame thereby preventing casual opening of the door or other partially opened door conditions with respect to the door frame. The system is of a low-cost, simple construction which is quick and efficient in operation and use.

In summary,a bifold door system which includes a door frame having top and bottom walls is provided. A trackway is mounted on one of the walls. A pair of door panels hinged together along adjacent vertical edges are associated with the frame with one of the panels being pivotally mounted at its top and bottom to the frame to enable the panel to rotate between the closed position substantially flush with the door frame and an open position out of the plane of the frame.

The other panel has means thereon movably mounted in the trackway so as to permit the panel to be shifted between a closed position substantially flush with the door frame and an open position out of the plane of the frame. The hinged connection between the panel permits the panels to be opened to a substantially face-to-face interengagement and to bias the panels into a closed position substantially flush with the door frame and maintain a continuing bias to retain the door panel in the closed position thereby requiring a prede- LII terrnined force to open the door and permit passage through the door frame.

Finally, resilient retention means are on the trackway to allow shifting of the hinged panels to the open position and then retain the panels in substantially the open position until a predetermined force is applied at which time the retention means will release the panels and allow the bias means to automatically return the panels to the closed position.

With the above objectives, among others, in mind, reference is had to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a fragmentary perspective view of two adjacent bifold door systems of the invention in a common door frame; FIG. 2 is a fragmentary sectional view of the bifold door system of the invention taken along the plane of line 22 of FIG. 1;

FIG. 3 is a fragmentary enlarged perspective view of a hinge portion of the bifold door system of the invention;

FIG. 4 is a fragmentary enlarged perspective view of the opposite side of a hinge of the bifold door system of the invention;

FIG. 5 is a fragmentary top plan view of the trackway portion of the bifold door system of the invention and showing the position of the door being retained in the open position;

FIG. 6 is a side elevation view thereof;

FIG. 7 is a fragmentary top plan view of an alternate embodiment of the trackway of a bifold door system of the invention showing the doors being retained in the open position; and

FIG. 8 is a fragmentary side elevation view thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 depicts a pair of adjacent bifold door systems 20 and 21 with the system 20 in the open position and the system 21 in the closed position. Both systems are mounted in a door frame 22 which includes a top wall member 23 spaced from a bottom wall frame member 24. In this manner the vertical opening of the door frame is determined. When both system 20 and system 21 are in the closed position, they are substantially in alignment and flush with the opening fonned by the door frame including top wall 23 and bottom wall 24.

Each system includes substantially identical components which cooperate with one another in the same manner to open and close the system. A pair of panels 25 and 26 are positioned in adjacent relationship. In other words each panels has an inner edge proximity to the other panel and an outer edge distal from the other panel. The inner edge 27 of panel 25 is interconnected with the inner edge 28 of panel 26 by a plurality of spaced hinges 29 along the vertical travel length of the panels. As shown, for example, there may be four separate hinges 29 for each system 20 or 21. It is readily apparent that a fewer or a greater number of hinges can be applied along the vertical length of the panels depending on size door in other satisfactory embodiments.

Panels and 26 are fixed at their upper extremity to a trackway30. Any conventional means can be employed to mount trackway 30 to the undersurface of upper wall 23 of frame 22. Attachment of trackway to the upper wall 23 can be accomplished in a conventional manner such as by screws or nails. Trackway 30 extends substantially parallel to the upper surface of panels 25 and 26 when they are in the closed position and are flush with the door frame.

The bottom of panel 26 is pivotally mounted to the inner surface of bottom wall 24 by means of a fixed pin 31 extending upwardly from adjustable mounting plate 32 into an accepting recess 33 in the undersurface of panel 26. The location of recess 33 is adjacent to distal edge 34 of panel 26. Mounting plate 32 is mounted in any conventional manner to the undersurface of bottom wall 24. The weight of panel 34 assists in retaining it in position on pin 31 and clearance is provided between the outer surface of pin 31 and the inner surface of recess 33 to permit panel 26 to pivot about pin 31 between a closed position flush with door frame 22 and an opened position substantially perpendicular to the plane of door frame 22.

Similarly, extending from the upper edge of panel 26 at a point adjacent to distal edge 34 is a pin 35 which is fixed to the upper surface of panel 26 by means of an adjustable mounting plate 36 and which is adapted to be positioned in receptacle 37 mounted in a conventional manner in trackway 30. Appropriate clearance isprovided between the outer surface of pin 35 and the inner surface of receptacle 37 to permit panel 26 to pivot between the closed position flush with frame 22 and an opened position substantially perpendicular to frame 22.

The undersurface of panel 25 is disengaged from bottom wall 24 and free to be moved with respect to bottom wall 24 from a closed position flush with walls 23 and 24 to an opened position substantially perpendicular to frame 22. Mounted on the upper edge of panel 25 is a mounting plate 38 on which is mounted an upwardly extending roller 39 which is dimensioned to reciprocally roll through the interior channel of trackway 30 as will be discussed in detail below. Roller 39 is rotatably mounted on a flanged pin 40 affixed in a conventional manner to mounting plate 38. Both mounting plate 38 and mounting plate 36 are affixed in a conventional manner to the upper surface the door panels. Roller 39 is mounted adjacent to the distal edge 41 of panel 25.

An appropriate knob 42 is mounted at normal door handle level on each of the panels 25 and 26 so that when either of the knobs 42 is grasped and pulled out of the plane of frame 26 hinged panels 25 and 26 will be shifted from the closed position to the opened position with the interconnected hinged edges moving perpendicular to the plane of frame 22 as panel 26 pivots about pins 35 and 31 and panel 25 pivots with roller 39 moving along trackway 30 to a point adjacent to pin 35. In this position, the door panels are fully opened and are in face-to-face engagement substantially perpendicular to frame 22 thereby permitting access through frame 22.

Turning particularly to trackway 30 as shown in FIGS. 5 and 6 of the drawings, the trackway has a substantially U-shaped configuration formed by an elongated base 43 and two opposed side walls 44 and 45. Extending inwardly from the free edge of side walls 44 and 45 are a pair of opposed flanges 46 and 47 respectively. In this manner, the base, side walls, and flanges of trackway 30 form an interior elongated channel 48 to capture roller 39 extending upwardly from door panel 25.

Trackway 30 is mounted with its open side facing downward and the base affixed to the undersurface of top wall 23 of frame 22. Channel 48 is large enough to receive roller 39 with the undersurface of roller 39 in position to be captured by flanges 47 and 46 and the upper surface of roller 39 being captured by base 43. The rotating circumferential surface 49 of roller 39 engages with at least one of the side walls 44 and 45 and is captured by the side walls 44 and 45 in the horizontal direction.

A resilient retention structure is provided in the trackway adjacent to but spaced from theend of the trackway to which distal end 34 of panel 26 is connected. This retention structure 50 may take several forms. For example, as shown in FIGS. 5 and 6 it is provided by a deformed flap 51 which is resilient in nature and is deformed inwardly so as to normally extend partially into channel 48. Flap or spring 51 may be formed by making a cut in side wall 44 and flange 46. This cut 52 initially is in an upward direction and then in a lateral direction a predetermined distance. The cut then forms a flap 51 which may be deformed inwardly as'discussed above. By constructing trackway 30 of a resilient but relatively strong material such as a spring steel, the resultant effect can be attained.

In operation, when door panels 25 and 26 are shifted to the opened position and roller 39 travels along the length of trackway it will deflect flap 51 outwardly and can be extended beyond cut 52. Thereafter when the door panels are released, flap 51 will tend to retain roller 39 in a position adjacent to the end of the trackway and to keep the door panels 25 and 26 in the opened position. This position is shown in FIGS. 5 and 6 of th drawings.

The retentive action occurs because the deformed flap will tend to return to its normal position extending inwardly of channel 48. Thereafter a predetermined force will be required to once again defonn flap 51 out wardly and permit passage of roller 39 to return the door panels to the closed position.

An alternative to the cut-out portion which forms flap 51, could be a conventional leaf spring mounted on the inner surface of side wall 44 to act in a similar manner in permitting passage of roller 39 when desired and retaining roller 39 in position when desired.

A further alternative embodiment is shown in FIGS. 7 and 8 of the drawings. The trackway 30a in that further embodiment is substantially identical to the trackway 30 of the above discussed embodiment with the exception of the cut-out portion 52. In place of the cutout portion a rivet 53 is provided in the side wall 44a. The head 54 of rivet 53 is positioned to extend inwardly from side wall 44 into channel 48. Trackway 30a is is constructed of a resilient material such as spring steel so that when roller 39 is extended through channel 480 under sufficient force it will contact rivet head 54 and deflect wall 44a outwardly permitting passage of roller 39 and achieving the open position for door. panels 25 and 26. Thereafter when the door panels are released, with the resilient side wall having returned to its normal configuration rivet head 54 will block return of roller 39a at a point near the end of channel 30a and will thereby retain panels and 26 in the open position. Sufficient application of force will once again resiliently deform side wall 44a and permit roller 39a to pass back through channel a to a position where door panels 25 and 26 are in the opened position. It is naturally contemplated that a suitable substitute for rivet 53 can be employed to form the projection into channel 48a as long as trackway 30a is of a sufficient resilient nature to achieve the above discussed desired result.

FIGS. 3 and 4 of the drawings are directed to the specific features of each hinge assembly 29. Each assembly includes a central pintle 55. Pintle 55 has the conventional center rod terminated by top and bottom end caps 56 and 57 respectively. Mounted on the central portion of the pintle rod is a torsion spring 58. For purposes described below, a 270 or more torsion spring has been found to work effectively in the present system. The remainder of the space on the pintle rod between the end caps is substantially taken up by four separate rotatable pintle sleeve elements. Each separate sleeve element has a hinge leaf extending laterally therefrom for interengagement with a door panel surface. ln the embodiment shown, the bottom sleeve element 58 is connected to leaf 59 which is mounted to edge 28 of panel 26 in a conventional manner such as by screw 60.

Abutting against the upper edge of sleeve element 59 is sleeve element 61 which has hinge sleeve 62 extending therefrom. Hinge leaf 62 is mounted in a conventional fashion such as by screw 63 to the inner surface of the side wall of door panel 25. The upper edge of sleeve element 61 engages with the lower edge of torsion spring 58 to retain the spring in position from the bottom end of pintle 55. Extending end prong 64 of the bottom end of torsion spring 58 abuts against leaf 62 to apply a biasing force against leaf 62 and door panel 25.

The upper end of torsion spring 58 on the pintle road abuts against the undersurface of sleeve element 65 from which extends leaf 66. Conventional fastener means such as screw 67 can be employed to mount leaf 66 to the inner side wall of door panel 26. The upper extending spring finger 68 of torsion spring 58 abuts against leaf 66 and imparts an appropriate force to door panel 26.

Mounted between the upper surface of sleeve element 65 and the undersurface of cap 56 is the fourth sleeve element 69. Extending from sleeve element 69 is leaf 70 which is mounted in a conventional fashion to edge 27 of door panel 25. The mounting may be accomplished by conventional means such as screw 71.

In operation, when panels 25 and 26 are in the closed position and are flush with frame 22, the torsion spring imparts a continuous force to the inner surfaces of panels 25 and 26 as shown in FIG. 2. This action provides positive retention means to retain the door in the closed position. The force is facilitated by the 270 or more torsion spring 58 which has its fingers 64 and 68 bearing against hinges attached to panels 25 and 26 respectively. The arrows in FIG. 2 depict the direction of the force when the doors are in the closed position.

To open the doors, either of the handles 42 can be grasped and pulled away from the frame in a generally perpendicular direction which will cause door panel 26 to pivot about pins 31 and and, by means of the hinged interconnection with panel 25, cause door panel 25 to pivot into the open position with roller 39 travelling through trackway 30 toward pin 37. This action puts an additional force on torsion spring 58. When panels 25 and 26 have reached the fully open position, they will be substantially face-to-face and substantially perpendicular to the plane of door frame 22. In that position, maximum bias has been applied to torsion spring 58 and roller 39 has past the resilient retention means in trackway 30.

At that point, gradual release of the door panels 25 and 26 will permit roller 39 to come into engagement with the resilient retention means so that the panels are held in the substantially open position as shown in FIG. 1. The door will remain in that open position as long as desired permitting access through frame 22.

To close the bifold doors, it is merely necessary to apply a minimal force to panel 25 to move it into the fully opened position and then release it so that the momentum generated by the door as it returns to contact with the resilient retention means will be sufficient to deflect the retention means and permit roller 39 to continue past the retention means through trackway 30 until panels 25 and 26 are once again in the closed position flush with the door frame 22. The force exerted by the biased torsion spring 58 will assist in fully closing the door panels quickly and efficiently and as described above, will exert a continuing further force against the door panel to maintain them in a fully closed position. In this manner, there is no danger of a partially opened door arrangement whereby portions of the panels extend outside of the plane of the frame where they might interfere with normal activity in the doorway area. Furthermore, the positive closing action alleviates the danger of accidental partial opening of the doorduring activity in close proximitythereto. It should be kept in mind, that when door panels 25 and 26 are opened, it is merely necessary to open the panels beyond the resilient retention means and then merely let roller 39 rest against the retention means to prevent the door from reclosing. Thereafter, minimal pressure against the door panel 25 to shift it to the fully opened position and then letting it free will generate sufficient force to overcome the retentive forces of the resilient retention means and permit the door to return to the closed position. It is readily apparent, that the resilient retention means can be adjusted so that the desired forces are achieved for optimum operation to both hold the door in the opened position and readily permit release when desired.

Thus, the above objectives, among others, are effectively attained.

We claim:

1. A bifold door system comprising:

a door frame including top and bottom walls;

a trackway mounted on one of the walls;

a pair of door panels hinged together along adjacent vertical edges;

one of the panels being pivotally mounted at its top and bottom to the frame to enable the panel to rotate between a closed position substantially flush with the door frame and open position out of the plane of the frame;

the other panel having means thereon movably mounted in the trackway so as to permit the panel to be shifted between a closed position substantially flush with the door frame and an open position out of the plane of the frame;

means on the hinged connection between the panels to permit the panels to be opened to a substantially face-to-face interengagement and to bias the panels into the closed position substantially flush with the door frame and maintain a continuing bias to retain the door panels in the closed position thereby requiring a predetermined force to open the door and permit passage through the door frame; and

resilient retention means on the trackway to allow shifting of the hinged panels to the open position and then to retain the panels in substantially the open position until a predetermined force is applied at which time the retention means will release the panels and allow the bias means to automatically return the panels to the closed position.

2. The invention in accordance with claim 1 wherein there are two pairs of door panels with a trackway for each of the pair of door panels within the door frame arranged so that the panel pairs open in opposed directions and close into a plane substantially flush with the door frame.

3. The invention in accordance with claim 1 wherein the trackway has a U-shapedv configuration and is mounted to the top wall of the frame with the open side of the trackway facing downward, a pair of opposed flanges extending partially inward from the bottom rim of each of the opposed sides of the U-shaped trackway substantially along theentire length of the trackway, a roller rotatably mounted on a pin extending upwardly from the upper edge of the other of said panels, the roller adapted to be mounted in captured position in the trackway by being supported on its undersurface by the inwardly extending flanges and contained on its upper surface by the base of the trackway, and in rotating engagement with the side walls of the trackway so that movement of the other panel will be facilitated by rotation of the roller and longitudinal movement along the trackway in its captured position.

4. The invention in accordance with claim 3 wherein a portion of one side wall and inwardly extending flange of the trackway is cut away from the retainer of the trackway adjacent to one end thereof and is deformed inwardly into the normal path of travel of the roller in the trackway, and at least the deformed portion of the trackway being formed of resilient material so that a predetermined force exerted on the roller will cause it to deflect the deformed portion of the trackdirection until a predetermined force is exerted to deform the leaf spring and permit passage whereupon subsequent to passage, the leaf spring will return to its normal configuration extending into the channel of the trackway.

6. The invention in accordance with claim 1 wherein the trackway is an elongated member having a U- shaped cross section, the trackway being mounted on the top wall of the frame with the open side facing downward, a flange extending inwardly adjacent the bottom end of each side of the elongated trackway to form a channel for capturing the means on the other panel movably mounted therein, the means mounted in the trackway being a roller member cylindrical in configuration with the undersurface thereof in engagement with the flanges extending inwardly from the sides of the trackway and the upper surface of the roller being retained by the base wall of the U-shaped trackway, the circular circumference of the roller being in engagement with at least one side wall of the U-shaped trackway to facilitate rolling and movement thereof along the length of the trackway, the trackway being of a resilient material, a protuberance extending into the channel of the trackway at a point adjacent to one end thereof so that a predetermined force is required to move the roller past the protuberance by expanding at least one side wall of the trackway and permit passage of the roller over the protuberance.

7. The invention in accordance with claim 6 wherein the protuberance is formed by a rivet through the side wall of the trackway with the head of the rivet extending into the channel in the trackway.

8. The invention in accordance with claim 1 wherein the hinge connecting the pair of door panels includes a pintle, a first leaf rotatably mounted adjacent the bottom end of the pintle and extending laterally therefrom into mounting engagement with the edge of one of the panels, a second leaf rotatably mounted adjacent the upper end of the pintle and extending laterally therefrom into mounting engagement with the edge of the other of the panels, a third leaf on the pintle adjacent to the first leaf and extending laterally from the pintle into mounting engagement with a wall of the other panel, a fourth leaf rotatably mounted on the pintle adjacent the second leaf and extending laterally from the pintle into mounting engagement with a wall of the one panel, a torsion spring on the pintle with the upper end thereof abutting one of the second and fourth leaves on the pintle and the lower end thereof abutting one of the first and third leaves on the pintle so as to be captured therebetween and tend to bias the panels to the closed position.

9. The invention in accordance with claim 8 wherein the spring on the pintle is at least a 270 torsion spring.

10. The invention in accordance with claim 8 wherein there are four hinges spaced along the vertical surfaces of the panels. 

1. A bifold door system comprising: a door frame including top and bottom walls; a trackway mounted on one of the walls; a pair of door panels hinged together along adjacent vertical edges; one of the panels being pivotally mounted at its top and bottom to the frame to enable the panel to rotate between a closed position substantially flush with the door frame and open position out of the plane of the frame; the other panel having means thereon movably mounted in the trackway so as to permit the panel to be shifted between a closed position substantially flush with the door frame and an open position out of the plane of the frame; means on the hinged connection between the panels to permit the panels to be opened to a substantially face-to-face interengagement and to bias the panels into the closed position substantially flush with the door frame and maintain a continuing bias to retain the door panels in the closed position thereby requiring a predetermined force to open the door and permit passage through the door frame; and resilient retention means on the trackway to allow shifting of the hinged panels to the open position and then to retain the panels in substantially the open position until a predetermined force is applied at which time the retention means will release the panels and allow the bias means to automatically return the panels to the closed position.
 2. The invention in accordance with claim 1 wherein there are two pairs of door panels with a trackway for each of the pair of door panels within the door frame arranged so that the panel pairs open in opposed directions and close into a plane substantially flush with the door frame.
 3. The invention in accordance with claim 1 wherein the trackway has a U-shaped configuration and is mounted to the top wall of the frame with the open side of the trackway facing downward, a pair of opposed flanges extending partially inward from the bottom rim of each of the opposed sides of the U-shaped trackway substantially along the entire length of the trackway, a roller rotatably mounted on a pin extending upwardly from the upper edge of the other of said panels, the roller adapted to be mounted in captured position in the trackway by being supported on its undersurface by the inwardly extending flanges and contained on its upper surface by the base of the trackway, and in rotating engagement with the side walls of the trackway so that movement of the other panel will be facilitated by rotation of the roller and longitudinal movement along the trackway in its captured position.
 4. The invention in accordance with claim 3 wherein a portion of one side wall and inwardly extending flange of the trackway is cut away from the retainer of the trackway adjacent to one end thereof and is deformed inwardly into the normal path of travel of the roller in the trackway, and at least the deformed portion of the trackway being formed of resilient material so that a predetermined force exerted on the roller will cause it to deflect the deformed portion of the trackway and permit the roller to pass therethrough and, thereafter, the resilient nature of the deformed portion will return it to its initial position so that it will retain the roller in position until a predetermined force in the opposing direction causes the deformed portion of the trackway to deflect once again and permit the roller to travel in the opposed direction.
 5. The invention in accordance with claim 1 wherein the resilient retention means is a leaf spring normally extending into the channel of the trackway to normally prevent passage of means from the other panel in either direction until a predetermined force is exerted to deform the leaf spring and permit passage whereupon subsequent to passage, the leaf spring will return to its normal configuration extending into the channel of the trackway.
 6. The invention in accordance with claim 1 wherein the trackway is an elongated member having a U-shaped cross section, the trackway being mounted on the top wall of the frame with the open side facing downward, a flange extending inwardly adjacent the bottom end of each side of the elongated trackway to form a channel for capturing the means on the other panel movably mounted therein, the means mounted in the trackway being a roller member cylindrical in configuration with the undersurface thereof in engagement with the flanges extending inwardly from the sides of the trackway and the upper surface of the roller being retained by the base wall of the U-shaped trackway, the circular circumference of the roller being in engagement with at least one side wall of the U-shaped trackway to facilitate rolling and movement thereof along the length of the trackway, the trackway being of a resilient material, a protuberance extending into the channel of the trackway at a point adjacent to one end thereof so that a predetermined force is required to move the roller past the protuberance by expanding at least one side wall of the trackway and permit passage of the roller over the protuberance.
 7. The invention in accordance with claim 6 wherein the protuberance is formed by a rivet through the side wall of the trackway with the head of the rivet extending into the channel in the trackway.
 8. The invention in accordance with claim 1 wherein the hinge connecting the pair of door panels includes a pintle, a first leaf rotatably mounted adjacent the bottom end of the pintle and extending laterally therefrom into mounting engagement with the edge of one of the panels, a second leaf rotatably mounted adjacent the upper end of the pintle and extending laterally therefrom into mounting engagement with the edge of the other of the panels, a third leaf on the pintle adjacent to the first leaf and extending laterally from the pintle into mounting engagement with a wall of the other panel, a fourth leaf rotatably mounted on the pintle adjacent the second leaf and extending laterally from the pintle into mounting engagement with a wall of the one panel, a torsion spring on the pintle with the upper end thereof abutting one of the second and fourth leaves on the pintle and the lower end thereof abutting one of the first and third leaves on the pintle so as to be captured therebetween and tend to bias the panels to the closed position.
 9. The invention in accordance with claim 8 wherein the spring on the pintle is at least a 270* torsion spring.
 10. The invention in accordance with claim 8 wherein there are four hinges spaced along the vertical surfaces of the panels. 